1. Field of the Invention
The invention pertains to a self-pumping hydropneumatic spring strut with internal level control, especially for motor vehicles.
2. Description of the Related Art
Self-pumping hydropneumatic spring struts usually comprise an oil-filled working cylinder, subject to the pressure of at least one gas cushion, which is located in a high-pressure chamber and acts as a spring, the working cylinder being divided into two working spaces by a working piston carried by a hollow piston rod, and a piston pump, which is driven by the movements of the spring, thus conveying oil from a low-pressure chamber to the working space connected to the high-pressure chamber, the piston pump being provided with a flow connection, which connects the high-pressure chamber to the low-pressure chamber.
Motor vehicle spring systems which are equipped with level control for adjusting the vehicle body to a predetermined height are already known (see, for example, DE 100 19 531 A1). A spring system of this type consists of at least one piston-cylinder assembly comprising a damping piston provided with a working cylinder, a piston rod, and a damping valve; a pump of an oil reservoir; and corresponding feed and return lines leading from the oil reservoir to the piston-cylinder assembly and vice versa, wherein the pump, consisting of a pump piston and a drive, is installed in such a way that the pressure of the piston-cylinder assembly acts on the pressure and drive side of the pump piston. In the presence of a fully automatic level control system of this type, the vehicle body can be raised by the action of this electric pump and lowered again correspondingly by the use of a discharge valve. A system of this type is able to meet the demand for maintaining the vehicle body at any one of several different levels, but it suffers from the disadvantage that the system is expensive to manufacture.
Self-pumping hydropneumatic spring struts are also known (see, for example, U.S. Pat. No. 6,494,442) in which the level control of the spring strut is brought about by a piston pump, which is driven by the movements of the spring and thus conveys oil from a low-pressure chamber to the working space connected to the high-pressure chamber. This type of spring strut also has an actuating drive, which shifts the spring strut as a whole in the vertical direction. With the help of the hydropneumatic spring strut in conjunction with a separate actuating drive, this system makes it possible to maintain the vehicle body at various levels, but, because of the nature of the system, sufficient installation space must be available in the vehicle. An appropriate amount of outside energy must also be supplied.